1. Field of the Invention
This invention relates to the detection of a member interrupting one or more beams transmitted between an emitter and an associated detector in an irradiated field, and more particularly relates to touch input detection systems for determining the intrusion and the relative position of a stylus within an irradiated field generally adjacent to an electronic display and formed by a plurality of infrared emitter/detector pairs.
2. Description of the Prior Art
Detection systems comprising a plurality of light emitting and detecting pairs located around the periphery of an irradiated field have been employed in a number of applications. For example, infrared detector fields can be used as a safety interlock or an anti-intrusion system for preventing access to a certain area or as a safety barrier around a machine. One ever increasing use of such systems is as a touch input device to an electronic display. For example, an array of infrared light emitters and detectors can be positioned around the periphery of an electronic display such as a CRT or a flat panel display. A number of such systems employ an X-Y matrix of emitters and detectors with associated emitters and detectors being axially aligned in pairs. In such touch entry systems, the introduction of a stylus within the irradiated field in proximity to a portion of the display permits input for controlling the operation of a computer communicating with the electronic display and the touch input apparatus. An example of a touch input system utilizing opto matrix frames consisting of infrared emitters and detectors may be found in U.S. Pat. No. 4,267,443, "Photoelectronic Input Apparatus" , issued on May 12, 1981.
Conventional infrared or light "type" touch input devices employ opto devices such as LEDs, photodiodes, and phototransistors, which are normally intended for near-looking applications. A touch input system would be characterized as a far-looking application. Near-looking applications are those applications in which the emitter and detector are located in close proximity. The majority of the applications for conventional infrared opto devices emitters and detectors employ a separation of only a few centimeters. Many touch input systems position the emitters and detectors more than a foot apart, and some have a spacing of several feet. In near looking applications in which the opto devices are closely spaced, the total power output of the devices is more critical than the on-axis intensity. Even so, conventional LEDs produced in the same process may range in light output by a factor of up to 4 to 1. Conventional phototransistors may range in sensitivity by the same factor. Therefore, when randomly paired together to form emitter/detector pairs, the output signals can vary over a range of 16 to 1 or even greater. This wide range of output signal can create difficulties in processing the output signals to determine broken beams.
Careful preselection of available opto devices is one method of obtaining acceptable performance for touch input systems. Many of the opto devices from any manufacturing batch must be excluded as totally unacceptable for use in touch input systems. Thus, devices at the low end of the distribution must be totally eliminated. This preselection is based on total power output rather than on-axis intensity so that even the "better" opto devices may not be suitable for use in touch input systems. Thus, devices which may pass a selection process based on total power output may fail when the alignment problems associated with mounting the devices are combined with low on-axis intensity. Random pairing of devices will result in some devices producing below minimum output signals. Generally, the opto devices in a touch input matrix are mounted on printed circuit boards, and the complete assembly is tested. Experience has demonstrated that at least one pair out of the nominal 64 pairs will have an output signal below a minimum acceptable value. The detection, isolation, replacement and retesting of the unsatisfactory devices requires manual intervention and adds increased cost to the manufacturing process. Furthermore, the current manufacturing process, traced to the performance limits of conventional opto devices, precludes the ability to set up a fully automated high volume production line.
The ability to use commodity type opto devices without requiring prescreening and rework inherent in conventional touch input systems would greatly reduce the manufacturing cost of touch input devices and significantly improve reliability. Ideally, the use of the lowest cost opto devices, making an opto manufacturer's total yield distribution suitable for use in touch systems, would not only reduce the direct cost of the opto devices themselves, but would reduce associated rework problems and lead directly to high volume automated manufacturing of touch input devices.
The invention, as illustrated by the preferred embodiment described herein, does not require extensive preselection of conventional opto devices, and thus leads directly to such manufacturing improvement. Furthermore, other advantages will accrue based upon the use of the invention described herein. For example, touch input systems employing signal preconditioning in the manner described herein can be used to compensate for environmental contaminates such as dirt or dust which reduce the output signal of opto pairs. Signal preconditioning also allows for special treatment at the edges of an opto matrix device, where reflection has traditionally been a problem. Signal preconditioning also permits a reduction in the LED drive current, allowing the selection of low current and lower cost multiplexing devices. In addition, lower LED drive current will extend the life of the LEDs. Low drive current also reduces the effect of thermal shock on LEDs. Signal preconditioning will also allow the selection of the devices to be independent of the matrix size and dimensions and will permit the devices to be used in higher ambient light environments. Furthermore, maintenance of touch input systems will be improved since replacement of failed opto devices will not require a selection process to insure suitable system operation.